During the past 20 years there have been great developments in the scientific understanding of the role of nutrition in health and physical performance. Epidemiological and physiological studies have provided evidence that certain forms of dietary behaviour may be linked with an increased risk of developing disorders such as high blood pressure, coronary artery disease and some cancers. This has resulted in dietary recommendations that are intended to reduce the incidence of these disorders in the community. The science of nutrition in relation to sports performance has progressed from empirical studies investigating the effects of dietary manipulations, such as restriction and supplementation, to the direct investigation of the physiological basis of the specific nutritional demands of hard physical exercise. This review is based on the premise that it is "what comes out' rather than "what goes in', which provides the clues to ideal nutrition for athletic performance. Various aspects of the physical demands of athletic exercise are viewed as stresses that induce specific biochemical, and hence nutritional, strains in the athlete. Training is the predominant demand in the athletic lifestyle. This is characterised by acute bouts of high power output. During one hour of hard training an athlete may expend 30% of his or her total 24-hour energy output. These high power outputs have important implications for energy substrate and water requirements. Carbohydrate, specifically muscle glycogen, is an obligatory fuel for the high power outputs demanded by athletic sports. Muscle glycogen is a limiting factor in hard exercise because it is held in limited amounts, utilised rapidly by intense exercise, and fatigue occurs when it is depleted to low levels in the active muscles. Liver glycogen may also be exhausted by hard exercise and low blood glucose contributes to fatigue. High sweat rates are demanded during severe exercise and large water deficits commensurate with energy expenditure are incurred during extended periods of hard training and competition. Salt, potassium, and magnesium are lost in nutritionally significant amounts in the sweat, but vitamins and trace elements are not. Adaptive mechanisms protect athletes against electrolyte depletion. Iron loss in sweat may contribute to the iron deficiency seen in some endurance runners. Protein is degraded and amino acids are oxidised during physical exercise. Protein is also retained during muscle building training. Recent investigations indicate that the minimal protein requirements of athletes may be substantially higher than those for sedentary persons.(ABSTRACT TRUNCATED AT 400 WORDS)